3. Let X= {3n | n > 0) and Y = {3n | n ≥ 0). Prove that XC Y.
4. Let X = {n³ + 3n² + 3n | n ≥ 0} and Y = {n³ − 1|n > 0). Prove that X = Y.
39. Prove that 1+2+2²+...+2n = 2n+1 - 1 for all n≥ 0